I recently rebuilt the top end of my 07 yz450f, 20hrs ago on the meter. The Cylinder had no cross hacthing left, so I bought a ball hone and and honed the cylinder, put in a new piston and rings and timing chain, and had the head redone. maybe the cylinder was to far gone? It has begun blowing out oil from the breather. Are there any other things that can cause oil to blow out the breather besides worn rings/cylinder, I dont think its overfilled put in 1000cc of oil. The bike has tons of hours on it. Thinking if it does need a cylinder I might do the crank too and maybee big bore and stroker it to 500cc.